1. Field of the invention
The present invention relates to control and regulating circuitry for electronically commutated direct current motors (commonly known by the term "brushless"). Such regulating and control circuitry is often integrated monolithically in semi-conductor devices.
2. Description of the prior art
Brushless DC motors are well known and their use is increasingly widespread because of their characteristics of low electrical noise. Although a direct current motor, the brushless motor behaves like an alternating current synchronous motor inasmuch as it is equipped with "position sensors" which are used in order to commutate the current electronically through the phase windings. The rotor is a permanent magnet which is made to rotate by the rotating magnetic field obtained by commutating the excitations of the windings of the stator. The position of the permanent magnet rotor determines the commutation and can be detected by means of suitable sensors such as light emitters and photo-detectors, Hall effect devices etc. These position sensors are, however, relatively expensive devices which introduce further reliability problems into the systems.
By virtue of the presence of electromotive forces induced by the rotation of the permanent magnet on the stator phase windings of the motor, it is in theory possible to reconstruct such back emf signals and to use them in order to determine the position of the rotor, or to use such electromotive force signals suitably reconstructed in order to synchronize the commutation.
For example, in the case of a three-phase motor, taking into account that the emfs induced in the three phases of the stator oppose the respective voltages applied, the equivalent circuit in star-configuration is represented schematically in FIG. 1. The three signals V(AO), V(BO) and V(CO) can be obtained using three differential amplifiers. One input terminal of each differential amplifier is connected to the star-centre voltage V(O), and the other input terminal of the three amplifiers are connected to the voltages V(A), V(B) and V(C), respectively. When the three signals are observed on the oscilloscope, they have the appearance illustrated in FIG. 2.
As can be observed, when the commutation from one phase to another takes place, high frequency disturbances occur in the form of voltage peaks (spikes), but in spite of this the trend of the three signals can be considered approximately sinusoidal. In particular, the sine wave appears clearly defined and essentially free of disturbances in the section of crossing of the axis of the abscissas. These signals also contain a term constituted by the emf induced by the movement of the rotor and, if deprived of the disturbances caused by the commutations, could effectively be used in order to determine, through their analysis, the position of the rotor without the aid of any specific sensor. On the other hand, the use of analog techniques such as, for example, a variable-frequency low-pass filter, in order to filter the disturbances caused by the commutation of the motor poses problems of integration of the necessary passive and response components, especially at low speeds of rotation of the motor. Another problem is constituted by the fact that the signals also contain terms linked to the values of resistance and inductance (R and L) of the windings. This implies a specific design of the control circuit which takes account of the values of R and L of the motor. Another known solution, according to which the information supplied by a VCO (Voltage-Controlled Oscillator) is used in order to control the commutation of the phases of the motor, also has problems in terms of insufficient rapidity of the control system in adapting itself to sudden variations of speed of rotation of the motor which can take place. A forced temporary mechanical locking of the rotation, for example, can cause the loss of the position.